The need to resolve objects smaller than those visible to the naked eye has existed from the beginning of observational science and natural philosophy. Since the 19th Century, the fundamental limitations of employing visible light to realize enhanced optical resolution, while unwelcome, have generally been well understood. In practical terms, these limitations have typically involved the wavelength of light used, which generally defines the minimum scale of effective measurement this despite the best efforts of lens makers to perfect or otherwise improve the optical art. Consequently, conventional light-optical methods have increasingly resorted to smaller wavelengths with the trend in semiconductor fabrication, for example, being toward the deep ultraviolet in order to resolve sub-micron features below the diffraction limit of visible light. However, diffraction-imposed constraints are generally not unique to lithographic processes alone; analytical methods used in materials characterization studies (i.e., microscopies, polarization studies and various spectroscopic methods) also suffer from substantially similar limitations.
Near- and Far-fields
Near-field radiation (“NF”; also known as “evanescent radiation” or “forbidden light”) comprises radiation that does not propagate through space, but rather is localized “near” the surface of objects, while far-field radiation (“FF”; also known as “normal radiation” or “allowed light”) generally refers to propagative radiation. In general, all illuminated objects have both NF and FF components; however, most conventional spectroscopic methods generally avail themselves to diffraction-limited propagative components in the FF.
Near-field Microscopy
In 1928, Synge described but was not able to demonstrate sub-wavelength imaging using NF light. By 1972, Ash and Nicholls employed the NF for microwave microscopy. More recently, the development and subsequent implementation of Scanning Probe Microscopy (SPM), Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), as well as a variety of other methods, effectively set the stage for the application of the NF to optical techniques.
Scanning Near-Field Optical Microscopes (e.g., SNOM or NSOM) generally use sharp (e.g., sub-wavelength) probe tips in order to image at sub-wavelength optical resolution. See, for example, D. W. Pohl et al.,       “          Optical      ⁢                           ⁢      Sthetoscopy      ⁢                           ⁢      Image      ⁢                           ⁢      Recording      ⁢                                         ⁢                                       ⁢      With      ⁢                           ⁢      Resolution      ⁢                           ⁢              λ        20              ”    ,Appl Phys. Lett. 44, 651-653, 1984. The, spatial resolution that may be achieved is generally defined by the size and shape of the probe tip. SPM also more generally includes a growing number of high resolution surface microscopy techniques (including NSOM) that use non-metrically sharp probe tips maintained very close to or even touching the sample surface and scanned in the xy plane of the sample using a combination of, for example, piezoelectric transducers, stepper motors and/or the like. See, for example, F. Zenhausern et al., “Scanning Interferometric Apertureless Microscopy: Optical Imaging at 10 Angstrom Resolution”, Science 269, 1083-85, 1995; and U.S. Pat. No. 5,646,731 issued to Wickramasinghe et al. on Jul. 8, 1997.
Although NF light generally may not be used to directly image a sample, the interaction of NF light on a sample with a sharp sub-wavelength probe may be exploited to “scan” an image of the sample's surface. There are various approaches that have been used for this in the prior art. In one method, the sample is illuminated with FF light, either from the top surface (or, alternatively, with back-side illumination) which produces a NF on the top surface of a transparent sample. See, for example, F. Zenhausern et al., “Apertureless Near Field Optical Microscope”, Appl. Phys. Lett., 65(13), 1623, 1994. Introduction of a sharp sub-wavelength probe into the sample's NF scatters FF light from a surface area approximately equivalent to the terminal cross-sectional area of the probe tip. The scattered FF light propagates away from the probe site for subsequent detection by, for example, interferometric means. If the probe size is smaller than the wavelength of the light incident on the sample and the probe is xy scanned at a correspondingly fine scale over the sample surface, an image having finer resolution than the wavelength may be “built-up”. If the probe is made with a tip having a terminal cross-sectional diameter on the order of about 100 nm and scanned at 100 nm intervals, in principle, 100 nm optical resolution may be achieved—much better than conventional FF light-optical techniques. See, for example, E. Betzig and J. K. Trautmais, Science, 257, 189, 1992.
In most cases, conventional FF mechanisms of optical contrast formation are generally applicable to NF imaging, however at much higher spatial resolutions—the ultimate limit of which is generally not believed to have yet been well characterized. Theoretically, it may be possible to construct atomically sharp NF probes to achieve atomic resolution. Other theories are based on the well-known       2    ⁢    π    λspatial frequencies limits. In practical terms however, as the probe size decreases, the volume of NF light asymptotically decays to zero with a corresponding trade-off between resolution and sensitivity. Practical high-resolution NF microscopy methods thus require robust optical techniques as well as good probe designs.
Spectroscopy and Chromatography
In terms of spectroscopic applications, “imaging” (e.g., detection) systems in analytical chemistry and separation science, while generally diverse, embody near ubiquitous implementations of FF processes. FF spectrochemical techniques, such as absorbance, fluorescence and chemiluminescence methods, have been generally well known in the art with the vast majority of these methods adapted for single-point detection at or near the end of a chromatographic column. The separation mechanisms generally operate over a duration of time and length of column (e.g., chromatographic field) such that the sample is resolved into component analytes which may then be interrogated by a detector, usually at a fixed position in the column-flow based on, for example, the physical and/or chemical properties desired in order to yield an analyte signal recorded over the separation. The analyte output is thus collected in a chromatographic domain (e.g., “time domain”), such as the signal obtained, for example, in an electropherogram and/or the like. In other words, the chromatogram is a convolved reducible expression of the detection function F(d) (in most cases, a Dirac delta function δ) in combination with the separation function F(s).
Prior art spectroscopic methods have generally employed means for detecting small scale phenomena that typically involve the reduction of incident wavelength (DeBroglie or otherwise) in order to achieve enhanced spectroscopic resolution. While these approaches may be acceptable in certain FF (e.g., “radiatively propagative”) systems, the broader application of chromatographic and spectroscopic technologies presents previously unresolved problems, for example, with respect to imaging below the diffraction limit of available FF methods. Accordingly, a representative limitation of the prior art concerns inter alia the effective and efficient NF detection of sub-diffraction-limited scale phenomena in, for example, a convolved G (F(d),F(s)) detection/separation domain.